1. Field of the Invention
This invention relates to Josephson devices and more particularly to such devices having improved thermal cyclability and the method of producing same.
2. Description of the Prior Art
Josephson devices are well-known in the art and generally include superconducting electrodes and a tunnel barrier between the superconducting electrodes. The tunnel barrier is commonly an oxide of the base metal of the electrode, although other materials may serve as the barrier. The tunnel barrier is extremely thin, usually from about 1 to about 10 nanometers, through which Josephson tunneling current can flow at zero voltage, with two tunneling states: a pair tunneling state in which Josephson current exists at zero junction voltage and a single particle tunneling state in which current exists at a finite voltage. These devices can be used to carry out logic and memory functions.
The production of Josephson devices is generally predicated on the formation of thin films of the component metals and poses difficult problems since the devices must show good stability over repeated thermal cycling, i.e. cyclability, and the tunneling resistance must be low in order to permit high Josephson current. In particular, stability of such thin films to repeated thermal cycling has heretofore posed considerable difficulty. Devices are subjected to temperatures ranging from 350.degree. K. to 4.2.degree. K. during fabrication and operation which subjects them to high strain during thermal cycling due to differences in thermal expansion coefficients of the device electrode and substrate materials. This is particularly characteristic of metals such as lead which have a low melting temperature and are desirable for use as superconducting electrodes of Josephson devices. Relaxation of strain can occur readily in such materials resulting, for example, in formation of hillocks, i.e. protrusions from the metal film surface, or dislocation steps which can break through the tunnel barrier of limited thickness and cause shorts between the first (base) and second (counter) electrodes.
To avoid the problems attendant to hillock formation and reduced thermal cyclability, U.S. Pat. No. 3,999,203 teaches the use of intermetallic compounds in the superconducting electrodes to provide suppression of strain relaxation. Thus, thin layers of lead alloys with gold and/or indium show greater stability and permit increased device thermal cyclability. The films produced by the method of this patent have average grain sizes ranging from 0.2 to 1 micron for 0.2 micron thick films.
While the foregoing approach has resulted in some improvement in the thermal cyclability of Josephson devices, there still remains the need for even further improvements which will result in Josephson devices for even greater stability, particularly stability to thermal cycling.
Improved strain behavior was observed in finegrained lead films deposited at low substrate temperature, e.g. 77.degree. K. (M. Murakami, Thin Solid Films, Vol. 59, p. 105 (1979). Such lead films have been used as counter electrodes for Nb/Pb tunneling junctions (U.S. Pat. No. 3,649,356). Pure lead films deposited at 77.degree. K. have a grain size of about 0.25 micron for 0.2 micron film thickness and do not have sufficient chemical or thermal cycling stability for use in lead alloy Josephson devices. Josephson devices with niobium base electrodes have about three times higher junction capacitance per unit area and thus are less favorable for high speed switching applications.